When manufacturing sails, also for competition purposes, it is an essential requirement to combine quite a number of special characteristics such as low weight, good handling qualities, low permeability to wind, high tearing resistance, elasticity, low water absorptiveness, UV resistance and similar properties. Therefore, the ultimate goal sailmakers have in mind is to create an optimized woven fabric for sail manufacturing which purposefully features all these characteristics.
In sail manufacturing processes it has not yet been attempted hitherto to integrate in a well-aimed manner structures into the surface that improve the aerodynamic properties of the material. If sail membranes are made from sheets they essentially have a smooth surface. If these membranes consist of woven fabrics which is frequently the case with high-grade and large-size sails the surface of the sails is characterized by structures that reflect the inherent woven material structures, as the case may be concealed by sheets, modified by coatings or changed by bonding or fusion means. While these fabric structures have an influence on the aerodynamic properties of the sails and, if applicable, their permeability to wind and water absorptiveness, they are unsuited, however, to purposefully change the resistance to air which is also due to their quite coarse structuring.
Resistance-causing air flow or stream separation and micro-eddying will arise on both smooth as well as structured surfaces that are facing the wind. By purposefully creating microroughness on said surfaces the resistance can be reduced. Roughness in this context is particularly microroughness that brings down the degree of turbulence in the turbulence layer in relation to the surface of the sail. Such microroughness has been developed for aircraft construction purposes where as a rule it is provided in the form of parallel grooves or flutes arranged longitudinally to the direction of the approaching air flow.
In the manufacture of sails a distinction is made between sails used for sailing close to the wind the propulsion of which is produced by the differential pressure occurring between the windward and leeward side and those sails used for sailing downwind (wind astern) the propulsion of which is for the main part brought about by the pressure exerted by the wind. Sails used for wind astern operation shall be tight to air, have a high tearing resistance and strength to withstand tear propagation and, especially in the case of spinnakers, be made of a light-weight material, feature good haptic characteristics and are easily set.
It is thus the objective of the invention to propose a sail membrane having a surface structure especially suited for sailing with wind astern, which can thus be used for the making of spinnakers and gennakers.